Gum arabic is a natural polymer having a good emulsifying ability and exhibiting low viscosities even in high concentration solutions, and because of these characteristics, is widely used as an emulsifier in the food and pharmaceutical fields. However, it is also known that natural gum arabic, when used as is, does not exhibit satisfactory emulsifying properties.
Gum arabic is collected in various countries in the Sahara region of Africa and has a wide variation in molecular weight and composition of constituent components due to differences in the soil and climate in each habitat and age of the source tree. For this reason, functions of gum arabic in its original state delivered from the habitat are nonuniformity, and thereby the properties of application products using such gum arabic are not consistent (Williams, P. A. and Phillips, G. O., (2000) in Handbook of Hydrocolloids, pp. 155-168, Editors: Williams, P. A. and Phillips, G. O., Woodhead, London and New York). In the present specification, such gum arabic is referred to as simply “gum arabic” or “natural gum arabic” or “gum arabic (unmodified)”, as distinguished from “modified gum arabic” of the present invention.
In order to solve the problems, several methods have been proposed and studied to reduce as much as possible the variation of emulsifiability between product lots attributable to the variation of the properties of the gum arabic (unmodified) and to improve emulsifiability. For instance, one method comprises removing metal ions from gum arabic to obtain arabic acid and subjecting it to thermal modification to improve (modify) its emulsifiability (Japanese Unexamined Patent Publication No. 1990-49001), and another method comprises modifying gum arabic having a loss-on-drying of not more than 50% by weight by heating it at 60° C. to 140° C. for not less than 30 minutes to thereby improve (modify) its emulsifiability (Japanese Unexamined Patent Publication No. 2000-166489).
However, these methods have drawbacks that, in the course of heating, gum arabic browns, melts and adheres to form syrupy masses, or undergoes charring. Browned gum arabic causes serious problems when used as an emulsifier, especially in a product whose appearance is considered important, such as foods, cosmetics, and the like. Further, when gum arabic melts, it gets stuck to the vessel, which makes it difficult to remove. Furthermore, once a syrupy mass is formed, gum arabic is difficult to pulverize to a powder for use as an emulsifier, and is not readily soluble in water. As gum arabic chars, such problems as odor emanation, formation of insoluble matter such as charcoal, and blackening of gum arabic solutions are likely to occur.
In addition to the aforementioned documents relating to modification of gum arabic, documents relating to gum arabic include WO02/072862, Japanese Unexamined Patent Publication No. 1983-183701, “Mikio Nakamura, Pharmaceutics, Vol. 42, No. 1 (1982) pp. 25-29” and “Carbohydrate Research, 246 (1993) pp. 303-318”.